Let's imagine Terran is going for a Hellion rush (1-2 build, by which I mean build 1 Rax + 2 Factories, Rax builds Reactor, then 1 Factory is switched with the Rax and builds 2 Hellions while Rax is building a Tech Lab that will be switched off to the 2nd Factory to get a quick "Infernal Pre-Igniter" upgrade).

Sometimes, doing such a build, I detect the Zerg player building a Roach Warren. For Terran that means you need to get either Tanks or Marauders quickly. But here is a problem: your 2nd Factory is still in progress, or you have the 2nd Factory but your first Tank is still building. A good Zerg will have 4-6 Roaches under your wall by that time. Because of the add-on buildings, you also won't able to build enough Marines/Marauders to put into bunkers.

1 Answer
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Short answer: Scout, detect the roach build before the Roach Warren, and change your build as necessary. Alternatively, start with marine/marauder and change to your hellion build if you detect a non-roach build.

Long answer: Let's assume you detected the Roach Warren 10 seconds after it went up.

If you detect the roach warren early enough, you can immediately throw a tech lab up (25 seconds) on your Barracks and build one marauder (30 seconds) in the same amount of time it takes the Zerg player to build their Roach Warren (55 seconds).

0: Roach Warren goes up.

10: Roach Warren detected.

15: Barracks starts building a tech lab; 2nd Barracks starts to be built.

40: Tech lab complete; Marauder 1 started.

55: Roach Warren complete, Roach 1-3 started.

70: Marauder 1, new, larva, Roach 4 started.

82: Roach 1-3.

95: Barracks 2, tech lab started.

100: Marauder 2. Marauder 3 started.

102: Roach 4.

120: Tech lab complete; Marauder #4 started.

125: Marauder 3. Marauder 5 started.

If at this point you see 4 roaches on your doorstep, you'll have 3 marauders complete (assuming a rush takes 23 seconds from when their 4th roach came out).

4 roaches = 32 DPS, 580 HP

3 marauders = 40 DPS, 375 HP

Statistically-speaking, the roaches will win without stim. If you researched stim, it'll be very close. If you had a bunker, you'd be fine.

Now, if they wait until they have 6 roaches:

85: Roach 5 started.

100: Roach 6 started.

112: Roach 5 completed.

127: Roach 6 completed.

150: Marauder 4 completed.

155: Marauder 5 completed.

Given the rush of 23 seconds, 6 roaches would appear in between your 4th and 5th marauders completing.

6 roaches = 48 DPS, 870 HP

5 marauders = 66.65 DPS, 626 HP

You're slightly better off. Without stim, it's close. With stim, you win. With a bunker or two, you are fine.

Note that this is all theory. Going for a hellion rush and having to bail out on it is not good news. Wasted time, resources, and a sense of panic can cause you to falter in this position. Even if you're able to stick to the example above, you won't be in an excellent position. You'll be forced into trying to scout to detect when the Zerg player changes their build to counter it and you'll be in a position where you're constantly reacting rather than being proactive and causing the Zerg player to react.

The best solution is to detect the roach build before the Roach Warren drops and get a head start.

If you watch a standard roach opener, you'll notice that rather than expanding after their 13/14 pool, the Zerg player is forced to save the minerals, wait until 18 to throw out the roach warren, and build up minerals for roaches rather than an expansion. When you don't see that expansion after the pool, it's a good sign that Zerg is going with a roach build. This is especially true if you do not see an upgrade happening at the spawning pool right after it is done. The upgrade happening at the spawning pool, in combination with an extractor being built early, usually hints at a baneling bust build instead.

If you swap over early, you'll be in a much better position than you would be if you waited for the Roach Warren to start building. You'll be proactive, the Zerg player will be reacting, and you'll have the upper-hand.

You can also spot him going for gas early on as an indication of teching up
–
Ivo FlipseJan 31 '11 at 22:25

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@Ivo Flipse: Also true. This is an earlier indication of a vespene-heavy build which is a good indication that the Zerg player will be advancing into higher-tier units quickly.
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ShaunJan 31 '11 at 22:34

Can't a lack of an expansion and early gas mean both roaches, or 1-base Mutalisk? Given that a Marauder-heavy army will fare poorly against Mutalisks, is there a way to differentiate these builds, before seeing a Warren or Lair upgrade/Spire?
–
WikwocketFeb 1 '11 at 3:20

@Wikwocket: Such builds usually start with the extractor before the pool so that they have enough gas to perform the upgrade to a Lair right when the pool finishes. Of course, Zerg can build the pool first and mimic the roach build until they diverge at the Lair upgrade, which makes it a little more difficult to predict. It also slows down their progress to mutas, making it a little easier to deal with. Either way, at that point, their defenses are usually such that you can reasonably scout the change and react. You'll have some marauders, but can switch to marines before things get dicey.
–
ShaunFeb 1 '11 at 3:35

I would point out that your normally have 7 or 10 roaches being produced as soon as the roach warren hatches for a 1-base roach attack, and so when they hit you will only have 2 Marauders. If you have a bunker, some SCVs to repair and the odd extra hellion/marine for some extra DPS you should be fine though.
–
NickJul 2 '12 at 11:06